The present invention relates to a method for measuring the electrolytic ion concentration of a sample liquid (e.g., the body fluid of an organism) by use of an automatic chemical analyzer, and to a system for using the method.
With a conventional method, the concentration of ions (such as Na, K, and Cl ions) in the body fluid of an organism is detected as follows. A liquid, such as the body fluid, the ion concentration of which is to be detected, will be referred to as a test liquid hereinafter. The electromotive force of a test liquid with an unknown ion concentration is measured by use of an ion-selective electrode. To correct the electromotive force measured, a reference liquid with a known ion concentration is used. More specifically, the electromotive force of this reference liquid is measured once, before or after the electromotive force of the test liquid is measured. The difference between the electromotive force of the test liquid and that of the reference liquid is calculated. By plotting the calculated difference against a calibration curve, which is prepared beforehand, the ion concentration corresponding to the difference is obtained. In the conventional method, however, the electromotive force of the reference liquid is measured only once, before or after the electromotive force of the test liquid is measured, or before or after the electromotive force of a standard liquid for correction is measured during the process of preparing the calibration curve. Therefore, there may be a delay in the response of the ion-selective electrode or an abrupt drift in the electromotive force, due to changes in the ambient temperature of the ion-selective electrode. Due to these adverse effects, the electromotive force of the reference liquid and that of the test liquid cannot be measured very reliably. Accordingly, it is very difficult to accurately detect the ion concentration of the test liquid. In order to eliminate the adverse effects noted above, the temperature of the ion-selective electrode must be strictly maintained at a constant value.
As can be seen from the foregoing, in order to accurately detect the ion concentration of the test liquid, it is very important to prepare the calibration curve without adverse effects caused by changes in the ambient temperature of the ion-selective electrode. It is also important to accurately measure the electromotive force of the reference liquid as well as the electromotive force of the test liquid.